P $4\mathrm{.}3$ What Are the Differences Between Freon $12$ and Its Replacement

HFC$-134$a$?($From Roberts, $1995)$

Hydrofluorocarbon $134$a $(1,1,1,2-$tetrafluoroethane$)$ is used as a replacement for

Freon $12($see Problem $4\mathrm{.}2)$ for refrigeration applications. $($Why is such a replace$-$

ment necessary and what is the advantage of HFC$-134$a from an environmental

protection point of view?$)$ Some vapor pressure data for Freon $12$ is given in

Problem $4\mathrm{.}2.$ The vapor pressure data of HFC$-134$a have been determined very

carefully and are as follows:

$1,1,1,2-$tetrafluoroethane

$($HFC$-134$a$)$

$($a$)$ Determine the normal boiling points $($in $C)$ of these compounds from the data

provided.

$($b$)$ At what temperature $(inC)$ will they have an equal vapor pressure?

$($c$)$ Compare the $($average$)$ enthalpies $({}_{\text{v}\text{a}\text{p}}{H}_i)$ and entropies $({}_{\text{v}\text{a}\text{p}}{S}_i)$ of vapori$-$

zation of the two compounds at the temperatures calculated under $($b$).$ Can

you rationalize any differences you observe between the two compounds?

$($d$)$ Automobile air conditioners commonly operate at temperatures between $30$

and $50C.$ Are the vapor pressures of the two compounds significantly $($i$.$c$.,$

greater than $10\%)$ different in this temperature region?

Note: P$4\mathrm{.}3($c$)$ should refer back to part P$4\mathrm{.}3($a$),$ and not P$4\mathrm{.}3($b$).$